This invention relates generally to fluid flow around elongated bluff bodies. More particularly, this invention relates to methods and structures for reducing drag and the effects associated with drag, and suppressing vortex-induced vibrations caused by vortex shedding associated with elongate bluff bodies disposed in fluid flow streams.
When a bluff body (i.e., a body having a broad flattened or rounded front), such as a piling, chimney, off shore riser, support tower, or similar structure is placed in a fluid flow stream, a drag force is created by the flow and exerted on the body. Additionally, vortex shedding can occur which induces forces that can lead to undesirable vibrations. Such vortex-induced vibrations can, if unabated, lead to premature structural deterioration or failure. The costs associated with constructing or strengthening structures to effectively resist drag forces, and to abate or compensate for the effects of vortex-induced vibrations, can be significant.
It is known to attach fairings or other structures to elongated bodies to modify fluid flow around such bodies to reduce drag and vortex shedding. The use of staggered separation wires, helical strakes, collars, rings, and fibers attached to the periphery of the body have been known to effectively disrupt regular vortex shedding. However, such measures often increase drag which is disadvantageous.
Discussions of vortex shedding may be found in E. Naudascher, D. Rockwell xe2x80x9cFLOW-INDUCED VIBRATIONS an Engineering Guide,xe2x80x9d IAHR-AIRH, Hydraulic structures design manual, A. A. Balkema/Rotterdam/Brookfield/1994, 160-176 and M. M. Zdravkovich, xe2x80x9cReview and Classification of Various Aerodynamic and Hydrodynamic Means for Suppressing Vortex Shedding,xe2x80x9d Journal of Wind Engineering and Industrial Aerodynamics, 7 (1981) 145-189.
A description of a unidirectional fairing for use on a drilling riser to reduce vortex-induced vibration is described in U.S. Pat. No. 6,048,136.
Another area where fluid flow around bluff bodies is encountered is in modern heat exchanger technology. Efficient and reliable operation of heat exchangers is determined in part by flow induced vibration of their fundamental elements. A common group of elements in such heat exchangers are bluff bodies that encounter a cross flow of air or other fluid. Such bodies may be circular cylinders (tubes), rectangular or elliptical pipes, and bodies of other geometric shapes. A circular cylinder is perhaps the most commonly used geometrical shape for elements in heat exchangers, power generators and other thermal apparatus. The circular cylinder is also a shape commonly used in boilers, steam and gas turbines, gas compressors and various other aerodynamic and hydrodynamic systems.
Circular cylinders, or tubes of similar cross-sectional shapes, are also used as structural elements in buildings, as pipe lines and, as previously mentioned, pilings, chimneys, off shore risers, and support towers.
There is a need for improved cross flow heat exchanger performance which may be achieved by reducing drag and suppressing vortex-induced vibrations in heat exchanger elements. There also exists a need to reduce drag and suppress vortex-induced vibrations in structural elements in many other of the applications cited above.
One aspect of the present invention provides a structural element which comprises an elongate body having a longitudinal axis extending along a wavy path. The body is coupled to a support on at least one end, and at least a portion of the body extends through an area of substantially unidirectional fluid flow. The body is oriented relative to the fluid flow such that a plane containing the wavy axis lies substantially parallel to the direction of fluid flow.
In one embodiment, the longitudinal axis extends along a substantially sinusoidal path. In other embodiments, the elongate body is coupled to a support at both ends. The portion of the body extending through the area of fluid flow preferably extends for multiple wave lengths through the flow area, and the wave steepness of the longitudinal axis is 0.05 or greater.
Another aspect of the invention comprises a method of reducing drag on and suppressing vortex-induced vibrations in an elongate body disposed in an area of directional fluid flow. The subject method comprises the steps of coupling at least one end of the body to a support, forming a longitudinal axis of the body along a substantially wavy path, and positioning the elongate body in the area of fluid flow such that a plane containing the wavy axis lies substantially parallel to the direction of fluid flow.
In certain embodiments, the elongate body has a substantially circular cross section. In this and other embodiments of the subject method, the longitudinal axis may extend along a substantially sinusoidal path. The subject method may include additional steps of connecting a second end of the body to a structure, device or apparatus to be supported by the elongate body, or coupling both ends of the body between respective supports.